1. Field of the Invention
The present invention relates in general to an active pixel sensor. More specifically, it relates to an active pixel sensor with a shared readout structure.
2. Description of the Related Art
Charge coupled devices are well-known image sensing devices utilized in various applications. In addition to charge coupled devices, active pixel sensors are also applied as image sensors. Active pixel sensors work by using photodiodes in conjunction with NMOS transistors (fabricated by the standard CMOS process) to sense images (light intensity).
FIG. 1 illustrates a circuit diagram of an active pixel sensor. In FIG. 1, the drain and source of a NMOS transistor T1 are coupled to a constant voltage source VB and the cathode of a photodiode Dp, respectively, and the anode of the photodiode Dp is coupled to a ground. The drain and source of a NMOS transistor T2 are coupled to the constant voltage source VB and the drain of a NMOS transistor T3, respectively, and the gate of the NMOS transistor T2 is coupled to the cathode of the photodiode Dp.
The active pixel sensor depicted in FIG. 1 utilizes the photodiode Dp to sense light intensity (images) and transforms it into electric signals, and then the electric signal is outputted from the source of the NMOS transistor T3. The operation of the active pixel sensor is described in detail as follows in reference to the timing chart of FIG. 2. For brevity, the waveforms in FIG. 2 are not plotted in actual voltage amplitudes and time lengths.
In the horizontal blanking interval, the NMOS transistor T3 is turned on by a selecting signal SL. In Co time interval (1), the NMOS transistor T1 is not turned on, and the voltage (VIN) at node A is amplified by the NMOS transistors T2 and T3, thereby obtaining a voltage V1 from terminal read.sub.-- out (the source of the NMOS transistor T3). In time interval (2), the NMOS transistor T1 is turned on by a reset signal RST, and a light-induced current generated by the photodiode Dp flows from the constant voltage source VB through the NMOS transistor T1 and the photodiode Dp to the ground, thereby charging the voltage at the node A to VIN'. In time interval (3), the NMOS transistor T1 is turned off, and voltage VIN' at node A is amplified by the NMOS transistors T2 and T3, thereby obtaining a voltage V2 from terminal read.sub.-- out. The difference between the voltages V2 and V1 corresponds the light intensity sensed by the photodiode Dp.
In general, an image-sensing device is implemented by using a plurality of active pixel sensors arranged in a two-dimension array of columns and rows. The IC layout of the image sensing device is schematically depicted in FIG. 3, wherein the regions enclosed by dashed-lines represent the active pixel sensors. For brevity, only a part of the conductive lines are shown in this figure, such as the selecting signal line SL, reset signal line RST, voltage source line VB, and output line read.sub.-- out. Based on the CMOS process, in every active pixel sensor, the selecting signal line SL and the reset signal line RST are fabricated by polysilicon lines, and the voltage source line VB and output line read.sub.-- out are fabricated by metal lines. Referring to FIG. 1 and FIG. 3, the voltage source line VB and output line read.sub.-- out are connected to the drain/source regions (n-type diffusion regions), and the source of the NMOS transistor T1 is connected to the gate (polysilicon gate) of the NMOS transistor T2. Consequently, fabricating an active pixel sensor requires three contact regions, three NMOS transistors, and a photodiode.
In view of the active pixel sensor described, the light intensity sensed by every photodiode is outputted via a readout circuit formed by transistors T1, T2, and T3. In an image sensing device, if two photodiodes disposed at two adjacent active pixel sensors can share the same readout circuit, then the chip (circuit) area required for fabricating the image sensing device and the process complexity can be reduced.